Born in 1654, he was the son of Robert Paston, 1st Earl of Yarmouth and his wife, Rebecca, née Clayton and was educated at Trinity College, Cambridge. He succeeded his father as 2nd Earl of Yarmouth in 1683, inheriting his estate and Oxnead Hall.
Paston was elected Member of Parliament for Norwich from 1678. In 1679, when his father was made an earl, William adopted the style of Lord Paston. He continued to represent Norwich until he inherited his father's title.
He converted to Roman Catholicism and in February 1687, James II appointed him Treasurer of the Household. He was also appointed joint Lord-Lieutenant of Wiltshire and Custos Rotulorum of Wiltshire in 1688.
Yarmouth died heavily in debt on 25 December 1732 at Epsom, Surrey, aged seventy-eight. In 1671, he had married the widowed Charlotte FitzRoy, the illegitimate daughter of Charles II and Elizabeth Killigrew. They had four children who survived childhood, though only one survived him:
- Charles Paston, Lord Paston (1673–1718)
- Lady Charlotte Paston (1675–1736), married Thomas Herne.
- Lady Rebecca Paston (1681–1726), married Sir John Holland, 2nd Baronet.
- Hon. William Paston (1682–1711)
As his sons, his brothers and their male heirs had predeceased him, his titles became extinct. His heavily mortgaged estate had to be sold.
- "PASTON, Hon. William (c.1654-1732), of Oxnead, Norf. and Turnham Green, Chiswick, Mdx". History of Parliament Trust. Retrieved 21 May 2019.
- "Fellows details". Royal Society. Retrieved 21 May 2019.
- John Miller, Paston, Robert, first earl of Yarmouth (1631–1683), Oxford Dictionary of National Biography, Oxford University Press, Sept 2004; online edn, Jan 2008, accessed 26 May 2008